The methods usually used for controlling the capacity of a blower are (a) to control the rate of rotation of the blower, (b) to provide a control valve on either the discharge side or the suction side of a blower and to control the capacity by adjusting the valve, and (c) to control the capacity by returning part of the gas from the discharge side to the suction side. In (a) the control mechanism is complicated; in (b) power is wasted when the valve on the discharge side is used for the control and there is a danger of surging. Further, when the valve on the suction side is used for control, power is also wasted and also there is a danger that the pressure on the suction side will become negative. In (c) much power is wasted.